Useful Reading
Campbell, Biology 6th Ed - Chapters 29 & 30, pgs 575-594, 597-605
Campbell, Biology 7th Ed - Chapters 29 & 30, pgs 573-597, 605-607
Kingdom Plantae is made up of multicellular, photosynthetic eukaryotes that evolved from green algae. There are 10 Divisions within the kingdom, which differ with respect to morphology and reproductive anatomy. All plants have chloroplasts and cell walls, and go through an alternation of generations involving a diploid sporophyte and a haploid gametophyte. However, plants differ in the presence or absence of three important structures: a vascular system, seeds, and flowers/fruits. These adaptations aided plants in the colonization of land from aquatic ancestors, and surviving in the changing terrestrial habitats. The description below lists the different evolutionary steps in plant phylogeny:
1. No vascular system : Non-vascular plants
Division Bryophyta (mosses)
Division Hepatophyta (liverworts)
Division Anthocerophyta (hornworts)
2. Vascular system : Vascular plants
A. No seeds :
Division Lycophyta (club mosses)
Division Pterophyta (ferns, horsetails, whisk ferns)
B. Production of seeds :
1) No flowers : Gymnosperms
Division Coniferophyta (conifers)
Division Cycadophyta (cycads)
Division Ginkgophyta (ginkgo)
Division Gnetophyta (gnetae)
2) Flowers : Angiosperms
Division Anthophyta (flowering plants)
During the evolution of plants, alterations were made in the life cycle, including the increased dominance of the sporophyte stage and the reduction and protection of the gametophyte stage.
For good sites on reproduction and life cycles in plants, click here and here.
Non-vascular plants (bryophytes) originated over 400 million years ago and have changed very little since then. Extant bryophytes are classified into three Divisions: Bryophyta (mosses), Hepatophyta (liverworts), and Anthocerophyta (hornworts). These plants lack specialized cells for the transport of materials (vascular tissue). Absence of vascular tissue limits bryophytes to moist habitats and small size. Unlike all other plants, the gametophyte is the dominant stage in bryophytes. During sexual reproduction, motile sperm swim from the antheridium to eggs in an archegonium (both of these are known as gametangia). Once fertilized, the egg develops into a sporophyte. Inside the sporophyte head (sporangium), spores develop. Once released, spores are dispersed by wind and develop into a new gametophyte.
Reference the life cycle above when looking at the following images:
In the picture of Haircap moss (Polytrichum) below, note the green gametophytes and the large sporophytes projecting up from the gametophytes.
Haircap mosses have visible antheridia:
In other mosses, antheridia and archegonia are not as large. Below are images of an antheridium (above) and archegonium with an egg (below).
Antheridia
Archegonia
Below is an image of a moss sporophyte with developing spores.
Moss spores germinate and produce protonemata (filaments of cells), eventually developing into a new moss gametophyte. The image below depicts sequential stages of spore germination (left) and gametophyte development (right).
Division Hepatophyta
Liverworts are made up of flat, lobed thalli (singular=thallus, it looks like a liver). There are pores on the upper surface that allow gas exchange and are always open. They also have circular structures, gemmae cups, on their upper surface which contain gemmae. The gemmae are dispersed by rain and develop into new liverworts (asexual reproduction).
Cross-section of a liverwort thallus, showing rhizoids (left) and an air pore (right)
Liverwort gemmae cups
During sexual reproduction, liverworts develop structures called antheridiophores and archegoniophores. In these structures are the antheridia and archegonia which produce sperm and egg, respectively.
Liverwort antheridia (left) and archegonia (right)
The fertilized egg develops into a sporophyte in the archegoniophore. Spores are released and develop into new gametophytes.
For more on bryophytes (and ferns!):
Seedless vascular plants are well-suited to moist areas, but do not do well in arid habitats. They are still dependent on water for external fertilization and for development of the free-living embryo.
These plants have several derived traits that they share with the seed plants:
1) a vascular system which transports water, nutrients and photosynthetic products around the plant,
2) sporophytes as the dominant life stage, and
3) stomata.
However, they retain some ancestral features:
1) free-living gametophyte,
2) no seeds, and
3) motile sperm that require water for fertilization.
Most seedless vascular plants produce one type of spore (homospory) which develops into a gametophyte. Some seedless vascular plants are heterosporous – megaspores develop into female gametophytes and microspores develop into male gametophytes. Heterospory is a trait shared with seed plants.
Division Lycophyta (club mosses)
Club mosses used to be important members of the flora during the Carboniferous period. The spore-producing sporangia are found either at the stem tips in strobili or along the stem at the leaf bases. Selaginella (shown below) is heterosporous and has sporangia in strobili.
Division Pterophyta (ferns, horsetails and whisk ferns)
Similar to the lycophytes, pterophytes were common in the Carboniferous period.
Whisk ferns are small and dichotomously-branched. Spores are dispersed by air.
Psilotum
Horsetails have jointed stems and large strobili at their stem tips.
Equistelum
Ferns are very leafy; leaves arise from an underground stem (rhizome). On the underside of some leaves (sporophylls), sporangia are clustered in sori.
Fern life cycle:
Spores are produced by meiosis and dispersed when the sporangia burst open. The spore develops into a small (often heart-shaped) gametophyte. On the underside of the gametophyte are the antheridia and archegonia. Motile sperm are released from the antheridia and swim through water to fertilize eggs in archegonia.
The fertilized egg develops into a sporophyte which grows out of the gametophyte.
Excellent Seedless Plant Website
Division Cycadophyta
Cycads are palm-like in shape are found in the tropics.
For more on cycads, click here.
Division Gnetophyta
Genotphyta contains gnetums (vines), Ephedra (Mormon Tea), and Welwitschia. Many are desert plants.
Ephedra (Mormon Tea) can be found in New Mexico.
Welwitschia Cool website
Division Coniferophyta
Coniferophyta includes the cone-bearing trees and shrubs, although not all cones are woody like pine cones. You are probably familiar with pines, firs, spruces, junipers, cypresses, and cedars. Conifers can be the dominant trees in many forests.
Pine cones (left) and juniper “berries” (cones, right)
As a gymnosperm, pines have advanced vascular tissue:
Pine leaves are needle-like (cross-section above)
Life cycle of a pine
As mentioned previously, pines are heterosporous, with microspores developing into the male gametophyte (pollen) within microsporangia and cones. Below is a pollen cone containing microsporangia, which are present only in the spring. Within the pollen grain, a sperm nucleus is produced.
The pollen is released from the microsporangium and disperses via wind to a megasporangium. You can see the pollen “wings” that aid in dispersal.
The megasporangium contains ovules and megaspores, which develop into the female gametophyte. Below is an ovulate cone. This gametophyte is maintained within the parent sporophyte tissue and produces eggs.
When a pollen grain lands on a megasporangium, it makes a pollen tube and fertilizes an egg with the sperm nucleus. The embryo develops inside the gametophyte, which itself is within a coating of sporophyte tissue. This structure is the seed.
Review Questions
- Name three challenges for plants of terrestrial life and provide at least one plant adaptation that functions to deal with each challenge.
-Why does a lack of vascular tissue limit bryophytes to moist habitats? to small body size?
-Are plant gametes haploid or diploid? Are they produced by meiosis or mitosis?
-For each of the following taxa, state whether the dominant life stage in the gametophyte or the sporophyte:
Fern
Moss
Conifer
Ginkgo
Liverwort
-What are produced by an archegonium?
-In the liverwort, is the sporophyte stage dependent on or independent of the gametophyte?
-Are gemmae cups involved in sexual or asexual reproduction?
-Name two land adaptations that ferns have which they do not share with bryophytes. Name two land adaptations which gymnosperms have but ferns do not.
-Where are sporangia located on whisk ferns, horsetails and ferns?
-Why is it that lycophytes and pterophytes were so large and abundant during the Carboniferous, but are so restricted now?
-What is the function of the wings on pine pollen?
-Is a seed composed of only sporophyte tissue or of gametophyte and sporophyte tissue?